The control interface between a pilot and the air control surfaces of an aircraft has received much attention over the history of aviation. The earliest control interfaces were very simple but required a great deal of pilot skill to operate. Modern “fly by wire” interfaces make use of a host of electronics to assist the pilot and make flight safer and more economical. In “fly by wire”, the pilot interacts with an electronic user interface that then controls actuators at each aircraft control surface. Designers are constantly working to make the control user interface safer, easier to understand and operate, and more effective, efficient and reliable.
So-called “closed loop control” using a “control law” is commonly used to control the aircraft during flight. The use of feedback control laws to augment the elevator command in the pitch axis of an aircraft has been used since the latter half of 20th Century. In terms of modern aircraft, digital control laws are used to implement control laws that use a reference command based on pitch rate, load factor or a combination of thereof. Airspeed in conjunction with a load factor may also be considered as a reference command. In some cases, all three variables are considered as reference command, that is, the load factor, pitch rate and airspeed are considered.
Typically in the aeronautical industry, “fly-by-wire” aircraft that operates in closed-loop in the longitudinal axis maintain the aircraft flight path while the pilot manually moves (deflects) a sidestick controller to provide a load factor command for most of the flight phases. In this case, a longitudinal control law provides neutral static speed stability and auto-trim. This type of control law provides excellent handling qualities while in cruise but not necessarily during landing. More specifically, longitudinal control laws often do not provide suitable landing flare characteristics, and in particular speed stability during landing. Generally speaking, landing flare is initiated by increasing the aircraft's pitch attitude during landing just enough to reduce the sink rate to a desired amount (e.g., 100-200 feet per minute) when the landing gear is a certain distance (e.g., approximately 15 feet) above the runway surface. In most jet airplanes, this will require a pitch attitude increase of a certain amount (e.g., 1° to 3°). The thrust ideally is smoothly reduced to idle as the flare progresses, but speed stability is especially important during this critical landing phase. See e.g., The Airplane Flying Handbook (U.S. Federal Aviation Administration 2011).
The standard solution in industry has been the utilization of radio altimeter sensor. The information of height above ground level is used to change the control law to a configuration with positive speed stability near to the ground. Thus, it has been the standard in industry that the configuration change in the longitudinal control law for the flare (that is, when the aircraft altitude reaches the flare altitude) is based on radio altimeter information. Particularly, a control law with neutral speed stability and auto-trim provides a control law with positive speed stability, when flare height is reached. However, this sort of solution has occasionally presented in-service events such as early flare activation during the approach due to erroneous height indication. Radio altimeter information can be corrupted by external and internal causes, such as water flow dirt or ice accretion on antennas, degraded connectors, reflectivity variations in terrain and contaminated runways. Another setback related to the usage of radio altimeter in critical flight controls application is the dependency on redundant sensors to guarantee the necessary system integrity. In other words, dispatching with one radio altimeter failed may not be possible to guarantee the necessary safety margins.
We have found that instead of or in addition to using height information, speed stability can be realized when using a longitudinal control law when aircraft is set to approach configuration, i.e. when the flap lever is set to the landing position and landing gears are locked down. This means that a change in the speed can only be accomplished while force is applied in the longitudinal pilot inceptor. Under such circumstances, the effort of trimming the aircraft speed can be extremely reduced by the usage of a momentary on-off switch in the sidestick, instead of or in addition to a conventional trim up-down switch, making easier the task of airspeed selection by the pilot. This control law provides good handling qualities during approach and landing, with the benefit of not needing or using radio altimeter information in safety-critical applications.
In an exemplary illustrative non-limiting implementation, a control law based on load factor control is presented. For example, the flight control law computes the load factor command based on a set of flight parameters and on the sensed position of the pilot inceptor. The pilot inceptor may be any of a plurality of devices used in aeronautics industry to serve as an interface with a human pilot, e.g. columns, mini-columns, central sticks, control yokes, or side-sticks. The flight parameters include, but are not limited to, in this example, flaps position, calibrated airspeed and dynamic pressure.
The technology herein aims to propose a flight control system and a method of adding positive speed stability characteristics to a longitudinal control law when the aircraft is set to the approach configuration, i.e. when the flap lever is set to the landing position and landing gears are down, without requiring use of radio altimeter information. The effort of trimming the aircraft speed during approach can be extremely reduced by the usage of a momentary on-off switch in the sidestick.
The exemplary illustrative non-limiting technology described herein is a flight control system that adds positive static speed stability to longitudinal control law when aircraft is configured for landing, i.e. flap levers in the landing position and landing gears down-locked.
Since the illustrative reconfigured control law for landing no longer provides auto-trim capability, a manual trimming process is performed similarly to a conventional aircraft: the pilot will be required to keep the longitudinal inceptor in a pulled back position in order to reduce the aircraft speed.
Once the target speed is reached, the pilot can set this new speed reference value by pressing the momentary on-off switch located in the sidestick, which reduces significantly the pilot workload. As long as the momentary switch is pressed, the reference speed is continuously resynchronized to the current airspeed. When the switch is released, the current airspeed is latched as a new reference.
In order to avoid transients in the primary surface, a rate limiter is applied while the new reference speed is still not reached by the aircraft. The reference speed may be indicated in the primary flight display as a speed bug in the speed tape. The engagement of the landing mode is indicated as a flag also in the primary display.
A non-limiting advantage of the illustrative solution is a control law that provides suitable handling qualities during both approach and flare flight phases. Therefore, the radio altimeter is no longer needed as a trigger for the flare control law. This eliminates the failure case of using erroneous height information and allows the dispatch of the aircraft with one failed radio altimeter without reduction of safety margins.
In one example non-limiting implementation, no additional hardware or physical parts are needed to implement the proposed solution when compared to the aircraft in the basic configuration.
An example non-limiting illustrative system provides a flight control system mode and method that provides aircraft speed control through the usage of a momentary on-off switch in the pilot inceptor. When configured for landing, the engagement of the proposed mode adds positive static speed stability to a longitudinal control law that controls a load factor demand. Such an illustrative system can provide:                A way to the flight control system detects that the aircraft is configured for landing. The flap lever, landing gear position and weight on wheels sensors can for example be used to characterize the landing phase. However, any other sensor used in aeronautical industry could be used to detect the flight phase, for instance, but not limited to, airspeed, inertial data, radio altimeter, or a cockpit switch activated by the crew.        A way to the pilot to change the aircraft speed when positive speed stability is engaged. In one proposed solution, the pilot will be required to keep the longitudinal inceptor in a pulled back position in order to reduce the aircraft speed and in the forward position to increase speed. The pilot inceptor may be any of a plurality of devices used in aeronautics industry to serve as an interface with a human pilot, e.g. columns, mini-columns, central sticks, control yokes, or side-sticks.        A way for the pilot to select a new reference speed. When the target speed is reached a momentary on-off switch located in the pilot inceptor is pressed to select the current speed as the reference speed. This momentary switch may comprise any of a plurality of devices used in aeronautic industry such as switches, buttons, rotating buttons, levers, touchscreens, etc;        A mean of processing data and computing outputs, based on a determined logic, and commanding the elevator surfaces;        A mean of commanding the elevator surfaces according to the command given by this mean of processing data and computing outputs;        A set of sensors which senses the configuration of the flight vehicle and the state of flight, to be used in a logic module that decides if the flight control mode is to be engaged and put into operation.        Once engaged, a set of sensors which senses the configuration of the flight vehicle and the state of flight, is used in a logic module that decides if the flight control mode should disengage.        